<!DOCTYPE html>
<html lang="en">
 <head>
  <meta charset="UTF-8">
  
 </head>
 <body>
  <script>
    var mycars = ["Saab","Volvo","BMW"];
    //从数组头部插入一个新元素，返回新的数组长度
    var newlen = mycars.unshift("奔驰");
    document.write(newlen);             //打印出数组新长度
    document.write('<br/>');
    document.write(mycars); 

  </script>
 </body>
</html>
